Environmental and fuel supply considerations have increasingly reordered priorities in designing engines which rely on burning fuel to produce power.
Internal combusion engines, driven by a reciprocating mechanism and operating on either Otto or Diesel cycles, have long been the popular choice for most applications. However, their combustion effluents and their reliance on petroleum fuels present increasingly difficult problems. Reviewed attention has therefore been directed to external combustion engines, which free combustion from the constraints of intermittent firing in a small space, and thereby open the way to improved combustion efficiency, wider choice of fuels, and better control of effluents.
For highest theoretical efficiency in external combusion engines, attention has long been given to Stirling cycle engines, which generate heat outside of a cylinder and conduct it through the cylinder wall to gas confined between two pistons in the cylinder. The heated gas powers the work stroke of one of the pistons to drive a crankshaft or the like. The other piston is driven by the shaft to aid in circulating the gas during the remainder of the cycle, including passing it through a heat absorbing regenerator and an external heat exchanger for cooling the gas. The Stirling cycle requires a closed body of gas which remains in gaseous state throughout, and operates in a sequence of heating the gas while at constant volume, followed by an adiabatic work stroke, cooling the gas while at constant volume, and adiabatic recompression of the gas before reheating.
The mechanical complications of Stirling cycle engine have led to development of other external combustion engines. Many of these avoid reciprocation by use of eccentric rotors in cylindrical stators and vanes extruding radially from the rotors to form a series of closed chambers between the vanes. The chambers expand in volume during half of each revolution of the rotor and contract during the other half revolution. The vanes can drive the rotor if gas at high pressure is supplied to the chambers as they begin to expand, as disclosed in U.S. Pat. No. 3,833 (Fletcher) issued in on Nov. 18, 1844. A closed body of gas for driving a vane type rotary engine is disclosed in U.S. Pat. No. 4,089,174 (Posnansky) issued May 16, 1978. Posnansky discloses use of solar power to heat the wall of a stator around a rotor carrying vanes forming closed chambers, where the chambers complete their contraction and begin expansion; and use of a radiator to cool gas withdrawn from the chambers while they are substantially fully expanded. After being cooled the gas is conducted back to the chambers while they are contracting. The Posnansky engine depends on the limited amount of heat that can be absorbed by a stator wall and transferred to a gas inside the stator next to the heated wall. Faster heating can be achieved by heating gas withdrawn from the stator and reintroducing it into such chambers as they begin to expand. This is disclosed in U.S. Pat. No. 3,774,397 (Engdahl) issued Nov. 27, 1973, but depends on the assistance of a pump and an operating cycle which involves condensation and vaporization of the gas.